This invention is concerned with the deposition of multiple layer optical thin films and, in particular, with techniques for continuously monitoring the thickness and refractive index of each layer in such films as it is deposited.
Optical films are of practical importance because they can be used to control the reflecting and transmitting properties of an optical system. A nonreflecting film, for example, can substantially reduce the loss of light by reflection at the various surfaces of a multielement camera lens. Stray light, which could otherwise reach the image because of these reflections, can also be substantially eliminated, with a resulting increase in contrast. Such improvements are particularly useful where an image is formed by a highly corrected lens system which employs a large number of interfaces between air and glass. Consequently, almost all optical components of high quality are equipped with thin film coatings to reduce reflection.
Thin optical films depend on the phenomenon of optical interference, which causes the intensities of transmitted and reflected light to be modified when two or more beams of light are superimposed. If a film of a transparent substance is deposited on glass, for example, with an optical thickness which is equal to one fourth the wavelength of a particular frequency of light, the reflection of that light from the glass surface can be almost completely suppressed by the quarter-wave layer. The light which would otherwise be reflected is not absorbed by a nonreflecting film; rather, the energy in the incident light is redistributed so that a decrease in reflection is accompanied by a concomitant increase in the intensity of the light which is transmitted.
Considerable improvements have been achieved in the antireflective performance of optical thin films by using composite films having two or more superimposed layers. The use of gradient index layers, in which the index of refraction within a layer is made to vary continuously as a function of depth in the layer, further increases the degrees of freedom available in the design of such films. In addition to these techniques, advanced optical thin film design procedures, which involve theoretically predicting the required refractive index profile for any desired transmission or reflection spectrum, have been instrumental in the development of a wide range of new optical devices whose performance is enhanced by spectrally complex filter structures. A rugate filter, for example, utilizes a gradient-index structure with a sinusoidal refractive index profile. The optical properties of the filter are determined by the values of n.sub.a, the average refractive index, and n.sub.p, the index modulation. As for quarter-wave reflectors, the width of the reflection band for such a filter is proportional to (n.sub.p /n.sub.a), while the peak value of the reflectance is determined by N(n.sub.p /n.sub.a) where N is the number of sinusoidal periods in the filter. High reflectivity can thus be maintained within a narrow bandwidth by increasing the number of periods in the rugate filter structure.
Practical realizations of rugate and other gradient index thin film structures, however, have been inhibited by the limitations of thin film fabrication technology. A highly accurate technique is needed to monitor in-situ the thickness and refractive index of the layers in these complex structures during the deposition process. Rugate filters, for example, have an allowable thickness error during deposition of no more than 1% of a layer's thickness. A slight change in the thickness of a single layer will introduce a phase shift which can have a significant detrimental effect on the filter spectral structure. Moreover, errors in refractive index within a deposition cycle will add additional frequency components to the profile, resulting in the growth of unwanted sidebands in the transmittance or reflectance spectrum of the filter. It is very difficult to compensate for such perturbations by the subsequent deposition of accurately fabricated layers. Consequently, precise monitoring of the deposition process is an absolute necessity.
The standard optical reflectance quarter-wave monitoring techniques of the prior art yield only the thickness of a deposited layer or, at best, the thickness and index of relatively thick layers deposited on known substrates. Generating gradient-index films with these prior art methods requires an exceptionally large number of monitor substrates. Consequently these conventional measurement methods have been found unsuitable to ensure the accurate deposition of complex gradient-index structures. A need has thus developed in the art for a monitoring method for these advanced thin film deposition processes which will measure the film thickness and refractive index without exhibiting the disadvantages of the standard methods.